1. Field of the Invention
The present invention relates to a zoom lens and optical equipment, and more particularly, to a zoom lens that favorably corrects the various aberrations of an optical system, in particular, the lateral chromatic aberration thereof by effectively using a diffractive optical element (diffractive optical surface), the zoom lens being suitably applied to optical equipment such as a lens shutter camera, a video camera, a digital camera, and the like.
2. Description of the Related Art
Recently, as the size of optical equipment such as a lens shutter camera, a video camera, and the like is reduced, a compact zoom lens having a high variable power ratio and a short overall lens length is desired as an imaging lens used in such optical equipment.
A so-called positive lead-type zoom lens, in which a lens unit having a positive optical power is the lens unit closest to the object, is employed in many cases as a means for reducing the size of an imaging lens. A remarkable feature of this type of imaging lens is that the length of the back focus can be reduced, which is effective for use in a lens shutter camera and the like that require no space behind a lens system to dispose a quick return mirror as used in a single-lens reflex camera.
Recent optical equipment employs a zoom type imaging lens composed of three or four lens units to cope with an additional requirement for a high variable power ratio, in addition to the requirement for reducing the size of the imaging lens.
Japanese Patent Laid-Open No. 06-250088, for example, proposes a zoom lens composed of four lens units which sequentially have positive, negative (or positive), positive, and negative optical powers from an object as a zoom lens composed of four lens units to simultaneously realize both a reduction in size of an optical system and the improvement of optical performance.
This proposal introduces a compact zoom lens the overall size of which is reduced by reducing the diameter of a front lens and which is excellent in optical performance by arranging a third lens unit as a focusing lens unit.
Similarly, Japanese Patent Laid-Open No. 06-265788 (corresponding to U.S. Pat. No. 6,002,529) proposes a zoom lens composed of four lens units having positive, negative, positive, and negative optical powers from an object.
The proposal introduces a simply constructed zoom lens having a high optical performance, which achieves these characteristics by regulating the shift amount of respective lens units in zooming and properly disposing aspherical surfaces.
As disclosed in the invention of Japanese Patent Laid-Open No. 06-265788, recently, various types of aberration can be relatively easily corrected by a smaller number of lenses due to advances in the technologies for making and designing an aspherical surface, which permits an imaging lens, which is small in size and excellent in an optical performance, to be obtained.
In the various types of aberration, however, chromatic aberration must be ordinarily corrected by the chromatic dispersion characteristics of a glass material constituting lenses and the combination of positive lenses and negative lenses. Thus, the correction of the chromatic aberration by an aspherical surface cannot be much expected.
Recently, a technology has been developed that corrects the chromatic aberration by a diffractive optical surface of a diffractive optical element disposed to a lens surface or a part of an optical system. For example, Japanese Patent Laid-Open No. 4-213421 (corresponding to U.S. Pat. No. 5,044,706), Japanese Patent Laid-Open No. 6-324262 (corresponding to U.S. Pat. No. 5,790,321), U.S. Pat. No. 5,268,790, and the like propose this technology.
Of these proposals, U.S. Pat. No. 5,268,790 proposes a zoom lens composed of four lens units having positive, negative, positive, and negative optical powers. In the zoom lens, which executes zooming by moving second and third lens units, diffractive optical elements are used in the second lens unit used for zooming and in the third lens unit for correcting variations of an image surface caused by the zooming.
In general, to increase a variable power while reducing the size of a zoom lens, there is a method of, for example, increasing the optical power of the respective lens units as well as increasing the amount of movement of the respective lens units used for a variable power.
However, a simple increase in the optical power of the lens units and in the amount of movement of the respective lens units increases variations in aberration, in particular, variations in chromatic aberration that are caused by the zooming. Thus, it is difficult to obtain an excellent optical performance over an entire variable power range.
While Japanese Patent Laid-Open Nos. 4-213421 and 6-324262 propose correcting chromatic aberration by the application of the diffractive optical element, they do not specifically describe a method of removing variations in chromatic aberration caused by the zooming characteristic to a zoom lens.
Further, the zoom lens disclosed in U.S. Pat. No. 5,268,790 performs zooming by moving the second and third lens units. Thus, it is difficult for the zoom lens to increase the variable power ratio while avoiding an increase in the size thereof. Furthermore, since the diffractive optical element is used in the second lens unit acting as a main zoom lens unit, chromatic aberration is multiplied or varied in zooming by the movement of the second lens unit acting as the zoom lens unit, and thus chromatic aberration cannot be effectively corrected.
Accordingly, it is at least one object of the present invention to provide a zoom lens and optical equipment using the zoom lens, wherein the zoom lens can obtain a desired variable power ratio, the size of which can be easily reduced, and which can favorably correct lateral chromatic aberration varied by zooming and has excellent optical performance over the entire variable power range by properly constructing the lenses of respective lens units and properly setting diffractive optical surfaces to be disposed on the lens units.
To achieve the above purpose, the zoom lens of the present invention comprises in sequence from an object side a first lens unit having a positive optical power, a second lens unit having a negative optical power, a third lens unit having a positive optical power, and a fourth lens unit having a negative optical power, wherein the zoom lens executes zooming by moving all of the first, second, third, and fourth lens units on an optical axis, and at least one of the first, second, third, and fourth lens units has at least one diffractive optical surface.
Further purposes, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.